Henry started a thread at
http://lists.w3.org/Archives/Public/public-xml-core-wg/2011Jul/0008
In the HTML5 spec, when dealing with the XML prolog, there is
nothing about processing instructions, so it isn't clear that
browsers would pick up the xml-stylesheet PI.
John Cowan said there was another part of the spec that applies
to that process, but Henry hasn't yet followed up on that.
ACTION to Henry: Research to ensure that the xml-stylesheet PI
is appropriately referenced from HTML5 and/or 3023bis (the XML
media type definition).
Note that 3023bis could say that browsers should reference
Assoc SS and process the SSPI when processing an XML resource.